Scavengers for radioactive iodine

ABSTRACT

A SURFACE COATING FOR ABSORBING RADIOACTIVE VAPORPHASE IODINE AND RADIOACTIVE VAPOR-PHASE METHYL IODIDE TO BE APPLIED TO THE INSIDE OF A CONTAINMENT VESSEL FOR A WATER-COOLED POWER REACTOR. THE ACTIVE INGREDIENTS OF THE SURFACE COATING MAY BE POLYACRYLAMINES AND COMBINATIONS OF POLYACRYLAMINES AND AMINE-TERMINATED POLYAMIDES. THE ACTIVE POLYMER INGREDIENTS ARE CROSS-LINKED TO PRROVIDE THE PHYSICAL CHARACTERISTICS NECESSARY TO WITHSTAND THE ENVIRONMENTAL CONDITIONS PRESENT IN A CONTAINMENT VESSEL SUBSEQUENT TO A LOSS-OF-COOLANT ACCIDENT.

United States Patent O 3,730,833 SCAVENGERS FOR RADIOACTIVE IODINE George E. Cremeans, Groveport, David A. Berry and Harvey S. Rosenberg, Columbus, Joseph M. Genco, Gahanna, and David L. Morrison, Columbus, Ohio, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Continuation-impart of application Ser. No. 803,465, Feb. 28, 1969. This application May 17, 1971, Ser. No. 144,298

Int. Cl. G21c 13/10 US. Cl. 17637 3 Claims ABSTRACT OF THE DISCLOSURE A surface coating for absorbing radioactive vaporphase iodine and radioactive vapor-phase methyl iodide to be applied to the inside of a containment vessel for a water-cooled power reactor. The active ingredients of the surface coating may be polyacrylamines and combinations of polyacrylamines and amine-terminated polyamides. The active polymer ingredients are cross-linked to provide the physical characteristics necessary to withstand the environmental conditions present in a containment vessel subsequent to a loss-of-coolant accident.

CONTRACTURAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This is a continuation-in-part from application S.N. 803,465, filed Feb. 28, 1969.

This invention relates to a method for removing radioactive vapor-phase iodine and alkyl iodides from the atmosphere. More particularly, this invention relates to a surface coating or paint which irreversibly reacts with vaporphase iodine or vapor-phase methyl iodide.

With the increased number of power reactors being built, it is inevitable that some of them will be placed close to population centers. Although the safety features of these reactors are impressive, research continues on methods for making these reactors even safer than they are now. One of the postulated catastrophes possible in a power reactor is a loss-of-coolant accident, hereinafter LOC, caused by the rupture of the main coolant line, i.e., a double-ended pipe break. One result of a LOC is the emission of a vast host of fission products. One of the most troublesome fission products because of its biological effects and long half-life is iodine 131. The iodine is present in the vapor-phase as elemental iodine gas and as vapor-phase alkyl iodides, predominantly methyl iodide. Exactly where the methyl iodide comes from is not certain, but recent studies seem to indicate that as much as 3,730,833 Patented May 1, 1973 20% of the iodine 131 released during a LOC may be present as methyl iodide.

Various methods are presently being developed to reduce the likelihood of vapor-phase iodine and vapor-phase methyl iodide escaping from the reactor after a LOC. The most common method is to 'spray a material into the core upon a loss of coolant or upon activation of the auxiliary coolant. The spray may contain boric acid or sodium thiosulfate or hydroxyl ion or it may contain additional chemicals which are also reactive with iodine. These sprays are classified as active safety systems for dealing with fission-product iodine.

It is the principal object of this invention to develop a passive safety system for removal, without any affirmative action, of vapor-phase iodine or vapor-phase methyl iodide from the containment-vessel atmosphere after a LOC. This and other objects of the invention are realized by coating the interior of the containment vessel with a paint that will irreversibly chemically react with vaporphase iodine and vapor-phase methyl iodide.

Paints generally contain three basic components: the binder or film-forming constituent, the pigment or filler and the solvent or, in the case of emulsion paints, an aqueous suspension medium. The primary function of the binder is to form a film or polymeric barrier over the substrate and to hold the pigment or filler in place. Heretofore, the binder has not generally been used as a chemically reactive material, but it is through the binders that the paints of this invention provide chemical functionality for reactions with vapor-phase iodine and vapor-phase methyl iodide. Theoretically, binders containing amines should provide the necessary chemical reactivity, but a complicating factor is the set of physical conditions which must be endured by any paint used for the annunciated purpose of this invention. During a LOC the containment vessel and hence the paint which covers it will be exposed to rather severe conditions and the paint to be effective must retain not only its physical integrity but its chemical functionality.

The principal criteria required of a coating or paint to function as a passive safety system are: (1) irreversible reactions with fission-product iodine from ambient temperature to C.; (2) a minimum required coating capacity for iodine and methyl iodide of 0.5 and 0.06 milligram per square centimeter of geometric area, respectively; (3) high deposition rates for fission-product iodine; (4) functionality in steam and under condensing steam conditions; (5) thermal and radiation stability; and (6) compatibility with active safety systems such as containment spray solutions.

SUMMARY OF THE INVENTION This invention comprises a passive safety system for preventing escape from the containment vessel of radioactive iodine after a nuclear reactor accident in which a paint containing a cross-linked polyacrylamine or a crosslinked combination of a polyacrylamine and an amineterrninated polyamide is applied to the interior surfaces of a nuclear reactor.

3 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The minimum coating capacities for iodine and methyl iodide contained herein are based on a 1000 MW(e) BWR containment vessel having a volume of 1.6 l ft. in the form of a sphere without internal compartmenting. The total iodine inventory was assumed at 12 kilograms of which 50% was assumed to be instantly released during an accident and 20% was assumed to be in the form of methyl iodide. Uniform iodine distribution in the containment vessel was assumed for the BWR and a PW'R containment vessel which, because it has a volume of 2.6x l0 ftfi, does not require as high iodine and methyl iodide capacities as does a BWR containment vessel.

Some of the various coating candidate systems postulated are vinyl pyridine-butadiene copolymers, epoxypolyamides, polyaminoacrylates, aromatic amines, as well as copper, nickel, iron and iron oxide used as reactive fillers and high-surface-area coatings such as foams, bubble coatings and coated porous substrates. 'It was found early in the testing program that there is no reliable correlation between reactivity of these various materials in solution and the heterogeneous reaction of vapor-phase iodine or vapor-phase methyl iodide with their materials in the form of coatings in the solid state. This made the choice of materials more difiicult because much prior experience with iodine absorption was inapplicable. Further, materials which have adequate functionality with respect to vapor-phase iodine often are inadequate with respect to methyl iodide. Although many amine-containing aromatics should theoretically be good scavangers and many were tested, the only aromatic which was found acceptable with respect to both vapor-phase iodine and methyl iodide and 1,lO-phenanthroline. This points up some of the difficulty in finding amines which actually react with vapor-phase iodine and methyl iodide.

The structures of only the most important systems are presented below:

1, IO-phenanthroline n=0,l,2,3,4-m. where the R and R are aliphatic groups and n=0,1,2,

Amine-terminated polyamide where R is an initiator fragment and where n is an integer 7 greater than 1.

Poly(dimethylaminoethylmethacrylate) (DMAM) LiiLJ where R is an initiator fragment and where n is an integer greater than 1.

Poly (t-butylaminoethylmethacrylate) (TBAM) The various coating candidates were initially tested under noncondensing conditions in which a steam-airiodine mixture (tagged with 1) having a constant iodine concentration was passed over a 25 mm. x 25 mm. glass specimen coated with the material of interest. The mass of iodine deposited was continually monitored by a scintillation probe located directly under the specimen. The iodine concentration in the gas phase was intermittently measured by collecting small samples of gas in a carbon tetrachloride scrubber and analyzing it for radioactive iodine. The air-steam-iodine mixture left the sample chamber and entered a steam condenser which removed the moisture from the gas and provided a quantitative measurement of the moisture of the gas stream. The mass of iodine sorbed and reacted by the coating specimen was obtained from the scintillation probe measurement as a function of time. The same method was also used to measure methyl iodide deposition rates by simply replacing the tagged elemental iodine with H tagged methyl iodide in the steam-air-methyl iodide mixture used for the methyl iodide experiments.

Various coatings were also tested under condensing steam conditions with both steam-air-iodine mixtures and steam-air-methyl iodide mixtures which were fed at a temperature of approximately C. into a chamber which contained several coated 1" x 1" specimens. The amount of steam condensing in the chamber Was regulated by controlling the temperature of the chamber a walls. A, portion of the vapor-phase iodine or methyl iodide partitioned between the water and the specimens while the remaining iodine or methyl iodide left the chamber in the outgoing gas stream. Provisions existed for monitoring the iodine concentration in the inlet and outlet gas streams and also in the condensate. Deposition of iodine on one of the specimen in the chamber was continuously monitored with a scintillation probe.

Numerous candidates were evaluated at 115 and C. for their iodine and methyl iodide capacities, degrees of sorption irreversibility and iodine and methyl iodide afiinities relative to a typical commercial coating, Phenoline 302, which is a paint containing amine functionality. All experiments were performed in approxi mately 50 v./o. steam-50 v./o. air atmospheres containing about mg. per cubic meter of either iodine or methyl iodide. Tables I and II show the mass of iodine or methyl iodide sorbed after one hour and 20-hour exposures together with the percentage or irreversibly sorbed material. It should be noted that after 20 hours exposure none of the experimental scavengers had reached saturation using a 2-mil thick coating in all cases. Reference to the tables shows that methyl iodide is more ditficult to sorb than is iodine and in an TABLE I.THE SORPTION OF ELEMENTAL IODINE BY VARIOUS POLYMERS AT 115 C. IN 50 VOL.

PERCENT AIR-50 VOL. PERCENT STREAM AT 1 ATM.

' I2 sorption at indicated exposure time, mg. I2 per Percent Ratio of 20 cm. I2 irrehrs. sorption versibly to that of Coating material 1 hr. 20 hrs. retained Phenoline 302 Phenoline 302 b 6. 33x10" 3. 04X10- 77. 4 1 Poly (amiuopropylmethylsilane) 6. 65x10- 2. 78 10- 81. 6 1 Versamid bubble coating c 1. 41x10- 4. 94Xl0- 55. 6 2 Versamid-30 wt. percent Epon d 1 64 10' 2. 24 67. 4 7 Polyvinyl alcohol-60 wt. percent dipiperidylpropane 3 93x10- 4. 9X10 92. 9 2 Epon-20 wt. percent dipiperidylpropane 2 06x10 2. 22 80. 1 7 Gentac 1 12X10- 2.32 71. 6 8 TBAM50 wt. percent nickel. 7 73X10- 1. 71 87. 6 6 TEAM 9. 49X1O- 1.75 93.6 6 DMAM 166x10" 3.09 93. 9 10 e The iodine concentration was 203 mg. per me.

11 An amine hardener by Carboline Corp.

0 A coating with amino and groups by General Mills.

* An epoxy cross-linking agent by Shell Chemical Corp.

6 A coating with amino end groups in the form of a pyridine moiety by General Tire and Rubber Co.

TABLE II.THE SORPTION OF METHYL IODIDE BY VARIOUS POLYMERS AT 115 C. IN 50 VOLUME PERCENT AIR-50 VOLUME PERCENT STREAM AT 1 ATM.

CHzI sorption at indicated exposure time,mg. CHaI Percent Ratio of per cm. 31 irrehrs. sorption versibly to that of Coating material 1 hr. 20 hrs. retained Phenoline 302 Phenoline 302 7. 47 l0- 6. 4.2)(10 84. 6 1 Poly(aminopropylmethylsilane) 1. 85 10 5. 74x10" 83. 0 9 Versamid bubble coating- 1. 01X1o-- 4. o6 10- 96. 5 7 Versamidwt. percent Ep 1 4l 10 1. 51x10+ 100.0 24 Polyvinyl alcohol-60 wt. percent dipiperidylpropane 2 30 10- 4. 69X10- 92.1 1 Epon-20 wt. percent dipiperidylpropane 3 64 10- 5 29 10 98. 1 3 Gentac- 2 70x10- 1 18x10 68.5 2 TEAM-50 wt. percent nickeL 5 59 10- 5 27 10- 97. 2 8 TBAM- 6 12 nr 7 29 10' 98.6 11 DMAM 1 64X10- 7 e2 10- 100.0 12 Genamid on asbestos b 3 61 10- 4 80X10+ 100. 0 75 1.10-phenanthro1ine on asbestos 1 02 10- 1. 22 92. 6 190 B The methyl iodide concentration was 174 mg. per

m. b The asbestos mats were about 28 mils thick and the composites contained about 25 wt. percent reactant.

efiort to increase the methyl iodide capacity nonfilmforming reactants were impregnated onto asbestos mats for evaluation under the same conditions as hereinbefore described.

Five coating systems for the sorption of iodine and methyl iodide were tested under condensing steam conditions and deposition rates were obtained therefor. In these experiments the condensation rate over the specimens was maintained constant at 5 10 gram/cm sec. The inlet iodine and methyl iodide concentrations were approximately the same as before. Table III shows some of the results of these experiments in which the steam-air-iodine or the steam-air-methyl iodide mixture entered the chamber at 115 C. and condensed on the chamber walls which were maintained below 100 C. For the experiment reported in Table III, the wall temperature was maintained at 37 C. as this represents the lowest deposition rate for these materials at the temperatures tested. Additional experiments at 60 and 90 C. indicated higher deposition rates and capacities at the higher temperatures.

TABLE III.DEPOSITION VELOCITIES AT 37 C. UNDER B Inlet iodine or methbl iodide concentration was about 175 mg. per m in 50 vol. percent air-50 vol. percent steam.

b 25 wt. percent phenanthroline on 28 mil thick asbestos.

e No detectable methyl iodide deposition.

Reference to Table III shows that coatings may be designed which have iodine affinities equivalent to those exhibited by the best asbestos composites and which have a significantly improved iodine affinity as compared to commercial coatings.

The 8 coating systems shown in Table III(a) below were tested for methyl iodide and iodine capacity in steamair environment over a temperature range of 37 to C.

TABLE III(a).-COMPOSITION OF SCAVENGER COATINGS (WT. PERCENT) B Cross-linln'ng agent; 1, o-dibromohexane.

b 1, lo-phenanthroline; Tradename for charcoal.

The above systems all contain cross-linking material in order to obtain adequate resistance to steam at 60 p.s.i.g. Specimens containing the various coating candidates were subjected to steam at 150 C. for 20 hours. It was found that the addition of pigment such as titanium dioxide, zinc oxide or activated charcoal had no effect as to the coatings ability to react with methyl iodide or as to their resistivity to steam. The addition of talc appeared to decrease the coatings reactivity with methyl iodide.

Tables IV and V report data for the eight coating on their capacities after 4 hours of deposition for iodine and methyl iodide.

TABLE IV.-COATING CAPACITIES FOR I2 IN MGJCM. AFTER 4 HRS. OF DEPOSITION Coating 37 60 90 115 140 170 TABLE V.COATING CAPACITIES FOR 01131 IN MGr./ClVI. AFTER 4 HRS. 0F DEPOSITION Deposition temperature, C.

Coating 37 G0 90 115 140 I70 *All values are multiplied by except the asterisked values which are multiplied by 10*.

As seen in the tables, the iodine capacities of all eight coatings exceed the minimum requirement of 0.5 mg./ cm. over the temperature range tested. Table V shows that some coatings did not even approach the required methyl iodide capacity of 0.06 mg./cm. at a temperature I hours. The coatings performed substantially the same;

about 10% of the iodine was lost and about 5% of the methyl iodide was lost irrespective of the deposition temperature of the coating. Desorption at 170 C. is a stringent test; the percent loss at lower temperatures would be less.

The effect of gamma irradiation on the deposition process was determined by exposing the coatings to 1 10 rads in a 5,000 curie cobalt-60 source at a dose rate of 7 x10 rads/ hr. The coatings were evaluated for elemental iodine deposition at 90 C. under condensing steam condition and compared to unirradiated coatings. The tests indicated that gamma irradiation had no adverse affect on the elemental iodine deposition.

The eight coatings, see Table III(a), were subjected to LOC environmental tests. The specimens were put into an autoclave and exposed to saturated steam at a pressure of 60 p.s.i.g. The pressure was lowered to 5 p.s.i.g. in 24 hours and held at that value for 12 days, followed by 14 days at 2 p.s.i.g. The specimens were immersed in various spray solutions held at 90 C. for one month. The last test was the simultaneous exposure of the specimens to steam at 105 C. and 10 rads gamma irradiation at a rate of 7 1O rads/hr. The results of these tests are reported in Table VI.

TABLE VI.LOC ACCIDENT ENVIRONMENTAL TEST RESULTS Test III spray solution Test IV 'y-radiation 1 e 2 B 3 e and steam M S S S M S S S U S S S U S S S U U U M U U U M M U U U M U U M Solution compositions are given below:

NorE. S=satisfactory performances; M=marginal performances; U=unsatisfactory performances.

The Genamid-epoxy coatings, 1a and 1b, passed all the tests with the possible exception of the immersion in 1.7 weight percent boric acid solution. The Genamidepoxy-DMAM coatings, 2a and 2b, passed both the radiation tests in steam and the spray exposures in basic solutions. They performed marginally in the autoclave but had unsatisfactory resistance to the acidic spray solution. For the most part, the TBAM-DMAM coatings, 3a, 3b, 4a and 4b, which had the highest afiinities for methyl iodide were unsatisfactory in the environmental tests.

It was determined that the failure of the TBAM- DMAM coatings in pressurized steam and/or spray solutions was due to degradation of the cross-linking between the monomers in the coating. The TBAM-DMAM coatings were modified by the addition of an epoxy crosslinking material, and the following systems were tested for iodine and methyl iodide deposition as well as their integrity under loss of coolant accident conditions:

(1) 47.5 w./o. DMA-M+2.5 w./o. epoxy+50 w./o. TiO (2) 45 w./o. DMAM-l- 5 w/o. epoxy+50 w./o. TiO' (3) 40 w./o. DMAM-[-10 w./o. epoxy+50 W./o. TiO';; (4) 47.5 w./o. DMAMTBAM+2.5 w./0. epoxy+50 w./o. Ti0 (5) 45 w./o. DMAM-TBAM+5 w./o. epoxy-l-SO w./o.

(6) 40 w./o. DMAM-TBAM-i-IO w./o. epoxy+50 w./o.

TiO

The above six systems all containing a 50 weight percent addition of white pigment calculated after the active ingredients have been added. Tables VII an VIII show some data for 4-hour methyl iodide and iodine deposition tests with the above coatings at 37 C., 90, 115 and 170 C.

TABLE VII.-COATING CAPACITIES FORMETHYL IODIDE Mg./C1n. AFTER 4 HRS. OF DEPOSITION Deposition, C.

Coating 37 37 I70 a All values X10.

TABLE VIIL-COATING CAPACITIES FOR IODINE IN Mg./C1:n. AFTER 4 HRS. OF DEPOSITION Deposition, C.

As may be seen by comparing Tables IV, V, V II and VIII, coatings 2 and 3 performed significantly better at 37 C. than any other coatings reported, while coatings 3, 4 and 5 showed greater methyl iodide capacity at 170 C. than other coatings. Coatings 1 and 4 did not exhibit the required physical characteristics and were dropped from the testing program.

Coatings 2, 3, 5 and 6 were then subjected to the same LOC environmental tests as reported in Table VI and achieved satisfactory performances in all tests except the extremely basic (pH=l3.2) spray solution test, see Table IX below.

TABLE IX.LOC ACCIDENT ENVIRONMENTAL TEST RESULTS B Condensing steam conditions.

'= Solution compositions are given below:

a Results at R; test will continue to 10 R.

NoTE.-S=satisfactory performances; M=marginal performances U=unsatisfactory performances.

Solution pH Composition 4. 9 1.7 wt. percent H3303.

9. 2 1.7 Wt. percent HaBOa, 1.6 wt. percent Na2SzO .5H2O, 0.6 wt. percent NaOH.

3 13. 2 1.7 wt. percent H3303, 6.0 wt. percent NaOH.

Since the DMAM-epoxy and the DMAM-TBAM- epoxy coatings passed all the environmental test except that extremely basic spray solution test, and they exhibited the highest methyl iodide capacity, they are preferred. Clearly, however, the combination of polyacrylamines such as DMAM 0r TBAM with an amineterminated polyamide, such as Genarnid, will produce an above-average coating if sufiicient cross-linking material is added to provide the required physical characteristics. The preferred cross-linking material is an epoxy resin but dihalides and other art-recognized materials are acceptable.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a system for producing power from nuclear energy which includes a water-cooled nuclear reactor disposed within a containment vessel, the combination with said containment vessel of a coating on the inside of said vessel which will remove vapor-phase iodine and vaporphase methyl iodide present in the containment vessel as the result of an accident, said coating containing a pigment, at least 10 weight percent of an epoxy resin cross linking material, and a poly (alkylaminomethacrylate) or a poly (alkylaminomethacrylate) plus an amine-terminated polamide having the following structure H2N--R-NHLC-R- NHJ C-R-C--NHR-NH2 wherein R and R are aliphatic groups and n is an integer greater than zero or equal to zero.

2. The combination according to claim 1 wherein the poly (alkylaminomethacrylate) is poly (t-butyl-aminoethylmethacrylate) or a copolymer of poly (t-butylaminoethylmethacrylate) and poly (dimethylaminoethylm ethacrylate) 3. The combination according to claim 2. wherein the pigment is titanium dioxide and the coating contains 20 weight percent poly (t-butyl-aminoethylmethacrylate), 20 weight percent poly (dimethylaminoethylmethacrylate), 10 weight percent of an epoxy resin cross linking agent, and weight percent titanium dioxide.

References Cited FOREIGN PATENTS 1,115,460 10/1961 Germany.

OTHER REFERENCES Rosenberg et al., Fission-Product Deposition Studies, Part II: Iodine Deposition On Painted Surfaces, Trans. American Nuclear Society, vol. 10, November 1967, pp. 716-7.

Berry et al., Coatings for Retaining Fission-Product Iodine and its Compounds, Trans. Amer. Nuclear Soc., vol. 10, November 1967, pp. 717-8.

Genco et al.: FissioneP'roduct Deposition and its Enhancement Under Reactor Accident Conditions, BMI-X-10229, pp. 52.

CARL D. QUARFORTH, Primary Examiner E. E. LEHMANN, Assistant Examiner US. Cl. X.R. 176-38; 252410 

